1. Field of the Invention
This invention relates to apparatus adapted for long-term subsurface disposal, such as wellbore tubulars and completion hardware, and, in particular, to replaceable and slide-on antennas for such apparatus. It is also applicable to electromagnetic telemetry used in subsurface communications.
2. Description of Related Art
Petroleum is usually produced from oil reservoirs sufficiently far below a gas cap and above an aquifer. As the oil zone is being produced and depleted, the gas cap starts coning downward and the aquifer coning upwards towards the oil-bearing zone. Such migration can adversely affect the extraction of petroleum by creating pockets that are missed by the producer and by saturating the oil deposits with water. As soon as either gas or water hits the well, its oil production usually ceases instantly.
FIG. 1a shows a deviated wellbore 70 drilled in an earth formation for the purpose of withdrawing oil from the reservoir. If conditions were perfect and the formation was homogeneous and isotropic, the interface between the oil and the water (i.e. the oil-water contact) would rise uniformly as the oil is depleted. In this case, the maximum amount of oil is produced before the onset of water production. However, in reality, this may not occur because of variations in formation properties along the horizontal wellbore, such as formation permeability, or fractures in the formation. FIG. 1b shows how water sometimes forces itself up, as shown by the water xe2x80x98conexe2x80x99 72, adjacent the deviated wellbore 70. Since water has a lower viscosity than oil, the water in the cone will flow into the deviated wellbore 70 over the oil along the deviated wellbore. Reservoirs are monitored for changes in saturation and early signatures of coning so that corrective action can be taken.
One approach to surveying and monitoring a reservoir is to deploy electrodes on the exterior of the casing. U.S. Pat. No. 5,642,051 (assigned to the present assignee) describes a casing, which has external insulation, electrodes, and cables for use in the completion. With such sensors mounted on the tubular, subsurface information such as reservoir pressure, temperature, flow rates, fluid fractions, sand detection, and chemical properties is acquired. These long-term/permanently installed monitoring systems have been developed to facilitate efficient reservoir management, well planning and resource exploitation. Wellbore apparatus for long-term monitoring are commercially available from companies such as ROXAR(trademark) (information available at http://www.roxar.com).
Downhole techniques have also been proposed utilizing slotted tubes or liners. A liner is a specialized tubular used in a completion method to prevent the wellbore from collapsing, and may also be used to prevent sand grains and other small particles from entering the wellbore and forming debris piles, which may restrict fluid flow. A liner is most often used in a horizontal well and within a single producing formation. It is an alternative to leaving the hole completely open (i.e., with no casing), when an open hole may collapse or become blocked with debris. See James J. Smolen, Production Logging In Horizontal Wells, SPWLA THIRTY-FIFTH ANNUAL SYMPOSIUM, workshop notes, Tulsa, Okla., Jun. 19, 1994.
Cross-well monitoring is another approach to monitoring changes in reservoir saturation. This technique is an outgrowth of radar experiments conducted in the early 1970s. See Michael Wilt, Exploring Oil Fields with Crosshole Electromagnetic Induction, SCIENCE AND TECHNOLOGY REVIEW, August 1996; See also Q. Zhou et al., Reservoir Monitoring with Interwell Electromagnetic Imaging, SPWLA FORTIETH ANNUAL LOGGING SYMPOSIUM, May 30-Jun. 3, 1999. With this technique, a transmitter is deployed in one well and a receiver is deployed in a second well. At the receiver borehole, the receiver detects components of the transmitted and induced currents for determination of the reservoir characteristics between the wells.
Measuring the electrical resistivity near a borehole has long been used to determine production zones in oil and gas fields and to map sand and shale layers. The electrical resistivity depends directly on porosity, pore-fluid resistivity, and saturation. Porous formations having high resistivity generally indicate the presence of hydrocarbons, while low-resistivity formations are generally water saturated. The resistivity measurement is made by emitting, from a transmitter antenna, electromagnetic energy that propagates through the formation. A receiver antenna receives the electromagnetic energy propagating in the formation and, responsive thereto, the phase and the amplitude of the electromagnetic energy are measured. When two receivers are employed, the phase shift and attenuation of the electromagnetic energy are measured between the receivers and the resistivity of the formation is deduced from the aforementioned phase shift and attenuation. In common practice, most resistivity tools use electromagnetic energy in the frequency range of hundreds of kilohertz to a few megahertz. A typical distance between a transmitter and receiver is generally less than one meter because of the high rate of attenuation of high frequency electromagnetic waves in many subsurface formations.
Electromagnetic energy is also used for short-range communication between downhole systems when it is difficult to establish a direct-wired connection. Electromagnetic signals between antennas placed on subsurface apparatus are used to relay data along the system. An apparatus using such a technique is described in U.S. Pat. No. 6,057,784. The antennas used for these communication techniques are generally of the type used for measuring formation resistivity. However, typical frequencies used in downhole electromagnetic telemetry systems tend to be in the range of a few kilohertz to tens of kilohertz. The lower frequencies are required to transmit electromagnetic energy distances of tens of meters between the downhole tools. The higher frequency electromagnetic energy used in most resistivity tools might be too attenuated in low resistivity formations. Hence, details of the low frequency antennas (such as the number of turns) can be different from the high frequency antennas.
Typical downhole tool antennas consist of coils of the cylindrical solenoid type comprised of one or more turns of insulated conductor wire. These antennas are mounted on a support and axially spaced from each other in the direction of the wellbore. Conventional techniques for placing the loop antennas on the support involve wrapping the coil windings around the support. U.S. Pat. No. 4,949,045 (assigned to the present assignee) further describes the assembly and implementation of conventional antennas on while-drilling apparatus. Other resistivity measurement techniques implemented with cased wellbores involve disposing the antennas on run-in tools which are disposed through the casing so that the antennas align with slots in the casing. These techniques are described in published U.S. patent application Ser. No. 2002/0079899 A1 (assigned to the present assignee and incorporated herein by reference).
A completion is typically made up of a large number of tubular sections. The tubulars (e.g. liners and casing) are generally metallic. However, fiberglass and other non-metallic tubulars have recently been implemented for well completions. The length of a single completion section is typically 30 to 40 feet. Hence, a 3000-foot long completion might have 100 such tubular sections, which are threaded together as the completion is tripped into the well. If the antennas are wound directly on the completion, many electrical connections will have to be made at the rig when running the completion into the well. Other disadvantages to directly winding the antennas on the completion are the cost and time consumption in the manufacturing process. The completion is sent to a specialty shop to wind the coils and over-mold them with rubber. Shipping and handling 30 to 40 foot sections of completion with integral antennas is also difficult and not amenable to local manufacture.
Thus a need remains for techniques for constructing antennas separately and independently from completion apparatus and for deploying these antennas on the apparatus, preferably at the field.
The invention provides a wellbore apparatus. The apparatus includes an elongated tubular adapted for long-term disposal within the wellbore; at least one arcuate shaped member adapted to function as an antenna and for disposal on the tubular, the arcuate shaped member being independently formed with respect to the tubular; and each at least one arcuate shaped member having a coil disposed therein.
The invention provides a method of deploying an antenna on a tubular adapted for long-term disposal in a wellbore. The method includes disposing an arcuate shaped member on the exterior of the tubular, the arcuate shaped member being independently formed with respect to the tubular and including a coil disposed therein; and coupling an electrical source to the coil.
The invention provides a system for deployment of antennas within a wellbore, the antennas adapted to transmit or receive electromagnetic energy. The system includes an electrical cable having the antennas coupled thereto; a tubular adapted for disposal within the wellbore and to receive each of the antennas; wherein each of the antennas comprises an arcuate shaped member independently formed with respect to the tubular; and each arcuate shaped member having a coil disposed therein.
The invention provides a method for deploying antennas within a wellbore, the antennas adapted to transmit or receive electromagnetic energy. The method includes mounting each of the antennas along a tubular adapted for disposal within the wellbore, each antenna being coupled to an electrical cable and comprising an arcuate shaped member independently formed with respect to the tubular, each arcuate shaped member having a coil disposed therein; and disposing the tubular within the wellbore.